Super-Earths are massive terrestrial planets that are fairly common in the Milky Way. While the name implies that these extrasolar planets would be similar to Earth, the name only refers to the mass and does not imply anything about the surface conditions or habitability. However, Northwestern University astrophysicist, Nicolas B. Cowan and Dorian Abbot, a University of Chicago geophysicist, report the odds of these planets having an Earth-like climate are much greater than previously thought.

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The researchers new model challenges the conventional wisdom, which says super-Earths would be a waterworld, with its surface completely covered in water. They conclude that most tectonically active super-Earths, regardless of mass, store most of their water in the mantle and will have both oceans and exposed continents, enabling a stable climate such as Earth's.

"Super-Earths are expected to have deep oceans that will overflow their basins and inundate the entire surface, but we show this logic to be flawed," he said. "Terrestrial planets have significant amounts of water in their interior. Super-Earths are likely to have shallow oceans to go along with their shallow ocean basins."

On Earth, water is constantly traded back and forth between the ocean and the rocky mantle because of plate tectonics, Cowan and Abbot said. The division of water between ocean and mantle is controlled by seafloor pressure, which is proportional to gravity.

Accounting for the effects of seafloor pressure and high gravity are two novel factors in their model of the super-Earths. As the size of the super-Earths increase, gravity and seafloor pressure also go up.

"We can put 80 times more water on a super-Earth and still have its surface look like Earth," Cowan said. "These massive planets have enormous seafloor pressure, and this force pushes water into the mantle."

Cowan and Abbot accede that there are two major uncertainties in their model: that super-Earths have plate tectonics and the amount of water Earth stores in its mantle.

"These are the two things we would like to know better to improve our model," Cowan said. "Our model is a shot from the hip, but it's an important step in advancing how we think about super-Earths."

The paper titled "Water Cycling Between Ocean and Mantle: Super-Earths Need Not Be Waterworlds" will be published Jan. 20 in The Astrophysical Journal.